Automatic control systems for industrial machinery and the like are, of course, well known. The most widely used automatic control systems are electrical and employ conventional circuitry including electro-mechanical relays, push-button switches, selector switches, limit switches and pressure switches and the like to initiate, control and terminate operation of the machine.
Just coming into prominence in recent years as an alternative to electrical systems are automatic control systems which employ pressurized fluid; i.e., either positive or negative (vacuum) pressure as a motive force. Fluid control systems of this type have the advantage of being virtually shock and explosion proof, requiring minimal shielding. Furthermore, the service life of fluid control system components, control valves, for example, far exceeds that of electrical relays and the like.
The automatic fluid control systems presently available are severely limited in complexity and flexibility, however, by problems heretofore thought to be inherent in their make-up and, consequently, insoluble. As a result, these fluid control systems have found relatively limited use compared to the wide-spread utilization of automatic control systems using electro-mechanical relays with isolated pole electrical contacts, pressure switches, etc., to perform logic and control power functions.
As will hereinafter be discussed in relation to the system embodying features of the present invention, the problems heretofore though to be inherent in fluid type automatic control systems reside basically in the construction and utilization of the control valves employed. Accordingly a brief discussion of basic, conventional valve constructions is a valuable preface to an understanding of the present invention and its advantages.
The most commonly used types of directional, fluid control, relay valves, for example, are spool valves, slide valves, and poppet valves. Spool valves and slide valves are shear-action devices incorporating a movable valve element which slides across fluid flow paths to control flow between adjacent ports. The poppet valve on the other hand, is a seating action device which includes a movable member adapted to seat and obstruct a flow path whereby flow between adjacent ports is controlled.
A spool valve utilizes a valve body containing a cylindrical bore having a plurality of cylindrical valve sealing surfaces spaced along its axis, with valve ports normally extending radially through the body into the communication with the bore between the sealing surfaces. The spool includes a plurality of cylindrical lands which selectively mate with the valve sealing surfaces in the bore as the spool is moved axially, so as to control the flow of pressurized fluid between the ports. The valve sealing surfaces (or the spool) might incorporate sealing O-rings, for example, to improve the seal between the spool and valve body bore or, in the alternative, a precision sliding fit may be established between the bore and the spool to limit leakage between the ports.
The slide valve utilizes a valve body having a precision, lapped surface with holes or slots intersecting the surface. The holes or slots lead to separate ports in the valve body. A movable element referred to as a "slider", having a precision, lapped surface, with slots or connecting holes in the surface, alternatively connects or blocks the flow of pressurized fluid between ports as the slider is moved relative to the body surface. Leakage between the ports is limited by the small amount of clearance afforded at the precision, lapped surfaces.
Finally, the poppet valve includes a body having an axial bore formed therein with annular seats in the bore between its opposite end ports. A spool which incorporates annular surfaces designed to coact with the annular seats in the body moves axially in the bore and selectively blocks flow between the ports.
Automatic control systems of the fluid type presently in use all employ standard valves of the aforedescribed character. The spool valve is most commonly used. As previously implied, however, these standard spool valves, slide valves, and poppet valves all have a common failing which places severe limitations on their use in an automatic, sequential control system and, accordingly, limit the type of logic which can be performed by conventional systems. Unless these standard valves are used as three-way valves, with the output passage of a normally blocked pressure path connected to a hydraulic tank, in the case of a liquid system, or to atmospheric pressure in the case of an air system, any leakage in the valve between the ports tends to build up pressure at the output port. The result is a false signal which may cause the system to malfunction. Because of this valve function requirement, present systems are limited in complexity and flexibility, as heretofore has been pointed out. This is one reason why a large percentage of the automatic control systems in present use are electro-mechanical systems.
It follows that thousands of trained technicians throughout the world are familiar with the abstract symbols and schematic diagrams used for automatic electrical control systems employing electro-mechanical relays. Because fluid control system valves and valve functions, as they are presently known, cannot be functionally interchanged with electro-mechanical isolated pole relays, pressure switches, limit switches, selector switches and push button switches and the like, the understanding and use of the fluid control systems by technicians trained in the conventional automatic electrical control systems has been extremely difficult, if not impossible. As a result, the utility and serviceability of present fluid control systems has been severely limited.